Re: CD 942 failure tonight

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Bart Anderson <classi6-at-classictesla-dot-com> 

Hi Mark,

The .15" was the total gap width, so .075" each side. I run mine at 0.1" 
per side without problems, so I don't see .15" as a problem at all. I would 
have expected the safety gap to fire unless it was way out there (all we 
know about it is the width).

Take care,
Bart

Tesla list wrote:

>Original poster: Mark Broker <mbroker-at-thegeekgroup-dot-org>
>I would consider .15" with a rotary gap to be "too wide."  I have the RSG 
>gaps set to about .030" each on my RSG, and with 5/32"diameter 
>hemispherical ends requires almost 90V input to start conducting.  But it 
>seems this is not the norm, even for people running ~3*Cres.
>(15/90 with 9 strings of 12 1600V, .056uF Panasonics - a voltage rating of 
>19.6kV!)
>
>When my RSG phase isn't in the sweet spot, the safety gap, which is always 
>"properly" set, starts firing like mad.  I can only imagine the peak 
>voltages such spikes would reach without the safety gap.  I also noticed, 
>when I checked them, that the 100W resistors in the "Terry Filter" were 
>considerably hotter than normal after even 15 seconds of such operation.
>
>The last failure we heard about happened when one string of caps wasn't 
>connected, placing the cap seriously close to a resonant value.  Is it 
>possible this happened?  Is it possible the NST is really, say, 15/120?
>
>Just some ideas that haven't already been asked.
>
>Mark Broker
>Chief Engineer, The Geek Group
>
>
>
>On Sun, 29 Feb 2004 16:07:23 -0700, Tesla list <tesla-at-pupman-dot-com> wrote:
>
>>Original poster: Bart Anderson <classi6-at-classictesla-dot-com> Hi John,
>>
>>Ok, if the rsg total gap is 0.15" or less, then it's not the problem. 
>>Phasing may be a possibility. You are correct, the safety gap should be 
>>firing if the voltage is climbing up (unless it's set too wide). What are 
>>you using for electrodes in the safety gap which is across the rsg?
>>
>>The caps rms current rating is excellent "if" circuit resistance is 3 
>>ohms. If it's more like 1 ohm, then your looking at 22 amps or so. The 
>>tempeture rise is then 6.7 per deg C and what you want to keep your eyes 
>>on. I like to see this value down around 2 or less.
>>
>>The cap bank voltage rating (18kV) is set near the NST's Vp (17kV), so 
>>there is basically no derating built in - it's on the edge (which many 
>>here claim to get away with and obviously do). When I built my Geek MMC, 
>>I went with 18 caps per string (36kV). For you to do this and keep the 
>>same capacitance, you would end up with 144 caps (8 strings). If you look 
>>at the cost, large capacitances and derating can get quite expensive, 
>>even for MMC's. The only way to reduce the cost for larger capacitances 
>>is to have individual MMC caps of larger value (maybe someday) or simply 
>>take your chances with less derating built in.
>>
>>If you get the same heating with a static gap, then it may be worth 
>>converting the 4 strings of 9 into 2 strings of 18 which drops the tank 
>>capacitance to .033uF. This will allow more primary turns (higher surge 
>>impedance),  a higher cap bank voltage rating (36 kV), and deg C is down 
>>to 1.5 at 1 ohm. It also puts you that much closer to resonance rise 
>>problems if an srsg is used. The .033uF is about the static gap LTR value 
>>and it would be prudent to run it first with a static gap to check 
>>heating. If you then run with an srsg, be aware you may risk the NST as 
>>it's twice as low as the srsg LTR value.
>>
>>I'm not sure what the happy medium is, but the wiring and caps heating up 
>>as you have mentioned tells me the rms current is  high and an indication 
>>that circuit impedance is low. That coupled with the low voltage rating 
>>on the cap bank sounds like a good probability of cap failure.
>>
>>
>>Note, a similarity to consider where the number of caps per string play a 
>>major role:
>>1) Cap voltage rating.
>>2) Individual cap temp rise.
>>
>>Both need to be considered for MMC's.
>>
>>Take care,
>>Bart
>>
>>
>>Tesla list wrote:
>>
>>>Original poster: "John Richardson" <jprich-at-up-dot-net>
>>>Hi Bart,
>>>
>>>
>>>
>>> > Original poster: Bart Anderson <classi6-at-classictesla-dot-com>
>>> >
>>> > Hi John,
msnip...